We have a male Betta and we recently got a new tank, went from a 1.5 gallon to a 3 gallon tank. We also added two female Betta's but they didn't get along and I removed them from the tank. He just sits near the filter or in the plants and doesn't move around as much. Could he be sick all of a sudden? Or does he miss the smaller tank? He swam around and was very happy before

Good that you removed the females from the male-Its not a good idea to keep both male and female Bettas together unless breeding.

In both the 1.5gal and 3gal filtered tank-how much and how often are the water changes, are these live plants, water temp, additives used, appetite and any other tank mates. How long have you had him, what fin type is he and what did you place the females in.

Hi.. I returned the females. I had no where to put them and they fought with each other. I'm very new at this. The plants are soft plastic, maybe I need to get new ones. He is a half moon betta. I don't have a heater. I didn't have one in the smaller tank and he was happy. I have a temp stick in there and it's at 70 degrees. Could this be the problem? I also never had a filter before and now I do. The strangest thing...when I showed him the old small tank, he came over and acted excited. Could he really miss the small tank? Very odd. The girl Betta fish were only in there for 1 hour and we wathed them and moved them when they would get too close. When we took them out, he began making bubble nests and then seemed depressed. Maybe it's the heater he needs??? Sorry.. I am so new to this

Oh and water changes once a week with drops added to the tap water...he doesn't seem to be eating all of a sudden. I'm going out to get a heater today. Any idea's for anything else I should get? I used the 5 and 1 test strip and everything looks normal. Maybe he just needs a heater? My friend has one in a tank with no heater or filter and her teenager never cleans the tank and yet 2 years later that poor fish is still alive... I would never be so cruel. I love Gill and so does my kids.

OH, now I know why some female bettas in the stores have tears and missing fins. Probably returned by the customer for fighting. That's the most likely reason. After they're healed up for a couple days in the backroom, they're returned to the store shelves.

Makes perfect sense.

I thought you had a heater? I would recommend trying to buy a heater that's adjustable.

It could be water temp related since this is a tropical fish it is best to try and maintain a somewhat stable temp in the 76-80F range.

Healthy Betta can tolerate cooler and warmer temps short term IME

Good that you are getting a heater, I would also recommend that you increase your water changes to twice weekly in the 3gal filtered tank...50% water only and 50% to include the substrate by vacuum or stir and dip method.
Give the filter media a good swish/rinse in the bucket of old tank water a couple of time a month with a water change to maintain good water flow.